1. Field of the Invention
The present invention relates to a method for effectively removing elemental mercury contained in emission gas by using chlorinated compounds, and more particularly, to a method for removing elemental mercury contained in emission gas by converting elemental mercury, which may be difficult to remove by using the prior art, to oxidized mercury, which can be removed easily, by using strong oxidizing agents, such as OClO, ClO, ClOO, Cl, Cl2, and transient Cl species produced by the reaction of NOx contained in emission gas with NaClO2 injected from the outside.
2. Description of the Related Art
In general, mercury is harmless to humans and the environment, since it is commonly present in the form of an alloy (amalgam) with other metals, or in an HgS form when reacted with sulfur in nature. However, when mercury is reduced to elemental mercury by the high temperatures generated by various combustions, for example, a steel making process, a sintering process, a coal-fired power plant or a waste incineration process, then elemental mercury is released in the form of emission gas and it may harm both humans and the environment. Poisonous mercury, such as elemental mercury, oxidized mercury and particulate mercury, reacted with various chemicals contained in emission gas are released into the atmosphere.
Particularly, the composition rate of mercury compounds contained in emission gasses generated by combustion depends on the amount of chlorine, the amount of sulfur and the combustion temperature of raw materials. In general, when the amount of halogen compounds and sulfur in a raw material are relatively high, the content of oxidized mercury is increased. However, elemental mercury, among mercury compounds generated by combustion, may be hard to remove as compared to oxidized mercury and particulate mercury, due to the low reactivity thereof. Therefore, attempts at oxidizing elemental mercury, which may be hard to remove by using the air pollution control facilities of prior art, into oxidized mercury, have been researched in the past.
Among the methods for converting elemental mercury into oxidized mercury, injecting chemicals available for oxidizing elemental mercury into high temperature emission gasses is more easily available for the conversion than others. U.S. Patent publication Nos. 2005/0255022 and 2008/0060520, and Korean Patent Laid-Open Publication No. 2007-0040752 disclose a method for removing elemental mercury by using hydrogen peroxide. Further, U.S. Patent publication No. 2003/0161771 discloses injecting molecular halogen or a thermo-labile molecular halogen precursor generating molecular halogen into emission gas by using a pyrolysis reaction and using chemicals, such as Ca(OCl)2, MgBr2 and KI3 for oxidizing elemental mercury. U.S. Patent publication No. 2005/0147549 and Japanese Patent Laid-Open Publication No. 2005-230810 disclose a method for injecting an ammonium salt, such as NH4Cl, NH4Br, and NH4I, into high temperature emission gasses at the rear end of a combustion chamber. U.S. Patent publication No. 2007/0202020 discloses the attempt to oxidize elemental mercury by injecting NH4Cl, HCl, and Cl2 into emission gasses.
In addition, U.S. Pat. No. 6,294,139 and Korean Patent No. 0367140 disclose a method for producing materials available for oxidizing elemental mercury by the reaction of NaClO2 with HCl contained in a scrubbing solution as the oxidation process of elemental mercury in a wet scrubber. Similarly, U.S. Patent publication No. 2002/0068030 and Korean Patent Laid-Open Publication No. 2002-0044537 also disclose using HClOx (x=1, 2, 3, 4) and salt as an aqueous solution of chlorine oxi-acids used in the oxidation process of elemental mercury by using a wet scrubber.
The various examples of prior art above relate to a method for oxidizing elemental mercury in high temperature emission gases. However, there are problems in that the prior arts make use of a wet scrubber, or the fact that when the temperature of emission gas is decreased, the reactivity of the generated HX (X═Cl, Br, I) with mercury is decreased, the generated HX can therefore be reacted with the unreacted NH3 again, and thereby the generated salt can block the gas pipe. Also, there is a problem in that using and handling HX and X2 (X═Cl, Br, I) injected as mercury-oxidizing agents requires close attention because HX and X2 are corrosive gasses, and need a special handling and containment apparatuses at additional cost.